Tourists circle a kiva in Mesa Verde National Park in southwestern Colorado.

The invaders arrive in the cool, early morning. The imposing sandstone edifices and towers, dotted with darkened windows, had been quiet and peaceful, a silent city hidden deep within a rocky tableland rising hundreds of feet above the Colorado desert. But now, the canyon echoes with voices and the sounds of feet tramping down worn, rocky footpaths.

The strangers, a group of 12, reach the village and gaze up at the carefully constructed buildings, some soaring two and three stories, to the ceiling of the canyon's overhang. The leader of the group steps forward and turns to face the others. "This is the Spruce Tree House," she says. As if on cue, the strangers pull out high-end digital cameras and glossy tourist brochures and begin ambling through the sandstone ruins. Another day has begun at Mesa Verde National Park.

As the tourists snap photos and scramble into reconstructed kivas, bubbling with a coffee-induced morning high, park ranger Lorisa Qumawunu gives a brief history lesson. The builders of these structures were a prehistoric people who lived throughout what's known as the Mesa Verde region, a setting of stark canyons, lonely mesa tops and sparse desert expanses north of the San Juan River in southwestern Colorado.

Probably beginning in the first millennium A.D., these people built increasingly complex communities here: wide roads across the desert, impressive dams that revitalized the landscape, and the tallest buildings in North America until the age of the skyscraper. In the mid-13th century, many relocated their homes from mesa tops and open plains to canyon rims and cliff-face alcoves, building the cliff dwellings for which Mesa Verde is known.

Fifty years later, the area was deserted.

Walking through the ruins, Qumawunu points out traces of original soot still visible on ceilings, pieces of plaster clinging to walls. She says it's no longer appropriate to call the former residents of these buildings "Anasazi," explaining that it's a Navajo word meaning "enemy ancestors." The accepted term is now "ancestral Pueblo people," reflecting the fact that they're related to the modern-day Pueblo people of New Mexico and Arizona. For Qumawunu, the name change is important, personal. She's from the Hopi Pueblo.

These explanations are lost on many of the visitors. Most are busy talking loudly among themselves, peering quizzically into sandstone rooms. "Where's the ping-pong court?" one asks loudly.

Despite this morning's ruckus, it's a quiet time of year at Mesa Verde, with the summer throngs thinned to a wintertime trickle. But this will soon change: 2006 marks the national park's centennial, with a yearlong celebration culminating in four days of 100th-birthday festivities in June.

As the tourists prepare to depart Spruce Tree House, one asks Qumawunu the question on everyone's mind: Why, after having invested so much work in this place, did the ancestral Pueblo people leave it all behind?

The park ranger's answer sounds well-rehearsed: "We can come up with so many thoughts about why they moved in and why they moved out. But no one really knows for sure."

But Qumawunu's rote answer isn't the whole story. The truth behind the Mesa Verde region's strange prehistoric depopulation is complicated and messy. After all, when you start digging in these parts, you never know what you're going to find.

'This can't be!'

In the high desert of Hovenweep National Monument, a dozen miles northeast of Cortez, down a maze of crisscrossing dirt roads and past ramshackle mobile homes and rolling sagebrush plains, lies Goodman Point Pueblo, a little-known ruin whose size puts the individual Mesa Verde cliff dwellings to shame. On this clear, windy morning, Kristin Kuckelman walks past low ridges of dirt and piles of dusty rubble scattered beneath gnarled juniper trees all that's left of a village that once supported hundreds of people. She's hunting for clues to help solve the Southwest's greatest archaeological mystery.

Kuckelman, her face still hinting at a deep summer tan beneath a wide-brimmed, fraying canvas hat, is senior research archaeologist at Crow Canyon Archaeological Center, a nonprofit organization dedicated to ongoing excavations in the area. This site is her baby. During the summer, she oversaw the removal of untold amounts of dirt, debris and artifacts from the ruins, the first professional below-ground research ever undertaken at one of the largest and best-preserved ancestral Pueblo communities in the Mesa Verde region.

Even with nearly 30 years of archaeology under her belt, Kuckelman wrestles with endless questions at places like Goodman Point.

In the mid-13th century, here lay a bustling, thriving village, the culmination of hundreds of years of habitation. There had never been more people on the land. Then, a few decades later, the village was abandoned, along with every other community in an area of more than 20,000 square miles, as thousands and thousands of people moved en masse to the far south. There's little evidence of ancestral Pueblo people living anywhere in the Mesa Verde region after 1300. Later, Native Americans in the region the Utes and the Navajo would say that ruins like Goodman Point Pueblo were full of ghosts.

Considering their history, maybe they are.

Photo by Paul Ermigiotti, courtesy Crow Canyon Archaeological Center

Kristin Kuckelman, senior research archaeologist at Crow Canyon Archaeological Center, oversaw the first below- ground research ever undertaken at a large, well- preserved Pueblo community in the Mesa Verde region.

The ancestral Pueblo people didn't have a written language; no one left behind a detailed account of their last days in the Mesa Verde region. But Kuckelman believes that if she looks hard enough at Goodman Point Pueblo, she can find this story written on the walls, on the floors and in the trash heaps.

There's a partially excavated kiva, a subterranean dwelling near the northwest corner, that could hold a chapter of the story. Standing over it, Kuckelman lifts the plywood covering that will protect the underground chamber over the winter and peers into the darkness. When this kiva was first excavated last summer, workers discovered prehistoric ash in the hearth, perhaps the remains of one of the last meals ever eaten in the village.

She believes that researchers will find little evidence of maize in the hearth ash, compared to the amount of maize refuse in rubbish pits nearby. This isn't a wild guess. Kuckelman and her co-workers noticed the pattern when they ran similar tests at a nearby ruin, Sand Canyon Pueblo. These findings are helping Kuckelman piece together a new theory about the flight from Mesa Verde.

Kuckelman believes that as more and more people settled in the region in the 13th century, they overwhelmed their food sources, such as deer and wild plants. As a result, they became increasingly dependent on maize crops, as evidenced by the ubiquity of maize in refuse pits.

But then, something wiped out their ability to cultivate their crops, as indicated by the limited maize remains in hearths. This could mean that Kuckelman has found more than just evidence of the last meals ever eaten by the ancestral Pueblo people in the Mesa Verde region. She's found a possible impetus for their leaving.

"The folks in this area had become very, very dependent on crops like maize. Ultimately, I think that system backfired and collapsed on them," she says.

But why did the system backfire? For a while, archaeologists thought they had the single answer: a great drought. Studying ancient wooden beams found in Mesa Verde ruins, beams whose tree rings captured the climate conditions of the prehistoric time period, scientists in the early 20th century announced there'd been a massive drought in the region from 1276 to 1299 a climatic disaster that seemed perfectly timed to cause the mass exodus.

But scholars are skeptical of single-factor explanations. Could one drought, no matter how devastating, be enough to depopulate an entire region?

For decades, no one had the hard evidence to challenge the drought theory. That changed 17 years ago, thanks to the work of a Ph.D. student named Carla Van West.

Van West, who now works for the nonprofit wing of a historic preservation firm in Albuquerque, N.M., still exhibits the fire and pluck with which she launched her pivotal anthropology dissertation research at Washington State University. "Everyone assumed there was a major catastrophe that caused all those areas to be depopulated," she says. "Open, close book that's the end of the story. And it didn't seem that simple to me."

Van West decided to reconstruct the damage the drought would have done, starting with the same evidence used to support the drought theory: tree rings. She compared modern tree rings to contemporary weather patterns, soil productivity levels and population numbers in the region, piecing it all together using digital geographic maps. Then she traced these patterns back through history, using tree-ring records from the late 13th century to reconstruct the weather, crop yields and occupation levels at the time.

It was long, tedious work, and the project stretched into years. Her colleagues told her to wrap up the dissertation, that it was just her "admission to the club," that it would end up collecting dust on a shelf anyway. But she wouldn't stop. And when the payoff came, it was big.

"[Expletive, expletive, expletive!] This can't be!" Van West remembers shouting one day in 1989, when her computer first printed out the results of her calculations. They showed that the drought hadn't been powerful enough to wreak havoc on all of the fields. Even in the driest years, there were still fertile mesa tops in the Mesa Verde region that could have and should have supported sizable populations.

The case of the mysterious migration has been reopened, and at places like Goodman Point Pueblo, scholars like Kuckelman are working to close it and this time, close it for real. If drought and other climate conditions destroyed the maize crop at Goodman Point, as Kuckelman's research suggests, why didn't the people move to another, more fertile area in the Mesa Verde region instead of migrating far to the south?

"You look at a landscape that had ancestral Pueblo people living off it for a millennium," she says. "To me, if everything was fine, if they had access to plenty of food and animals, why did they leave? There had to be another factor in there."

And she might know what it is.

A history of violence?

The skull was an unexpected find. In the early 1990s, Kuckelman and her Crow Canyon co-workers were excavating ancestral Pueblo towers and rooms that clung like mollusks to Castle Rock, a blunt pinnacle of sandstone jutting out of the McElmo Canyon floor in southwestern Colorado. They weren't expecting many skeletons and then they found a skull.

Photo by Paul Ermigiotti, courtesy Crow Canyon Archaeological Center

In digging through bookshelves, as well as ruins, Kuckelman has pieced together a story that could explain why the ancestral Pueblo people vanished from the Mesa Verde area.

It was just the beginning. By the end of the four-year excavation, crews had uncovered more than 1,000 bones, the remains of dozens and dozens of men, women and children. None were formal burials. The shattered limbs, smashed teeth, snapped noses and fractured skulls suggested a much less respectful fate.

And the gothic overtones extended beyond the bones. Castle Rock was an appropriate name for the village; it felt like a citadel. On one rock face, archaeologists found a painting of figures armed with shields, bows and arrows, with two of them standing back to back, as if surrounded by a faceless enemy.

Something had wiped out the villagers at Castle Rock, something terrible. Kuckelman and her colleagues believed they knew what it was. In the 1870s, a government survey team traveling through the area had been told an evocative Hopi tale about Castle Rock, an account later published in the New York Tribune.

Long ago, the story went, savage strangers had besieged the village at Castle Rock for a month. The attackers were pushed back, but at a heavy price: "... the hollows of the rocks were filled to the brim with the mingled blood of conquerors and conquered, and red veins of it ran down into the caon. It was such a victory as they could not afford to gain again, and they were glad, when the long fight was over, to follow their wives and little ones to the south."

These strands of evidence were propitious. A suggestion of violence had always lurked behind Mesa Verde archaeology, but no one had ever brought forward incontrovertible evidence of large-scale physical conflict. "Castle Rock was the first evidence of a large group of people, possibly a whole village, wiped out by a warfare event," says Kuckelman.

Crow Canyon excavators found something else at Castle Rock, but Kuckelman doesn't like to talk about it. Her reticence isn't due to any squeamishness; Crow Canyon has a policy of addressing controversial topics only through scholarly channels, such as peer-reviewed journals. And what they found at Castle Rock is as controversial as it gets.

Scholars and laymen alike have long characterized pre-Columbian North America as an egalitarian Eden filled with peaceful "noble savages." Many believe archaeologists chose to overlook evidence of tumult and turbulence at prehistoric ruins such as those in the Mesa Verde region.

"We sanitized the history and ignored the violence," says Wendy Bustard, museum coordinator for the Chaco Culture National Historic Park in New Mexico. "I think that our interpretations of the Pueblo peoples as peaceful people came out of the 1960s, when we as a people were looking for examples of peace."

Lately, there's been added incentive for maintaining this tranquil prehistoric picture. After decades of strained relations, Mesa Verde archaeologists, with Crow Canyon leading the way, finally began building tentative relationships with Native American communities in the 1990s. Announcing that these people's ancestors were war-loving would not have been a good peace offering.

Into this simmering political pot, a physical anthropologist named Christy Turner II tossed a grisly skull as well as a load of other gruesome bones.

After 30 years of research detailed in his and his late wife's 1999 work Man Corn (a translation of the Aztec word tlacatlaolli, roughly meaning "human stew"), Turner revealed that he'd discovered a dark secret long overlooked by his colleagues.

By looking for very specific evidence among skeletal remains cutting marks, unusual burn patterns, abrasions apparently caused by rocks used as anvils, fractures that exposed marrow-rich interiors, an absence of spongy bones like vertebrae, and a beveling of bone tips possibly caused by cooking these pieces in ceramic pots Turner said he'd found proof of 38 cases of cannibalism in the prehistoric American Southwest.

The outcry was immediate. Many scholars agreed with Turner that cannibalism had apparently occurred in the prehistoric Southwest, but they were concerned by Turner's dramatic ideas that these cannibals belonged to zealous warrior cults, possibly led by Charles Manson-like sociopaths. They worried that such lurid explanations would stir up unwanted media attention.

Sure enough, the headlines came fast and furious: "A Reign of Terror!" "Cannibals, Witches and War," "American Cannibal," "Ancient American Cannibals," "Cannibals of the Canyon," all disturbingly reminiscent of the xenophobia and racism directed at Native Americans that archaeologists had been working hard to overcome.

"I don't think any of us would like to have our ancestors called cannibals, but in a community that has suffered from archaeological incursion for so long, it's an even more sensitive issue," says Bustard.

Some scholars suggested that such a divisive topic should only be broached in the mainstream media if there was incontrovertible evidence of cannibalism, rock-solid proof. As Kurt Dongoske, a Turner critic and archaeologist for the Hopi tribe, put it in a 1996 National Geographic article, "As far as I'm concerned, you can't prove cannibalism until you actually find human remains in prehistoric human excrement."

In the mid-1990s, an archaeological team from the private consulting firm Soil Systems, Inc. began excavating prehistoric pit houses in a broad floodplain a few miles west of Mesa Verde National Park, a lonely place called Cowboy Wash. They soon uncovered piles of disarticulated bones scattered among the ruins.

In an article published in 2000, the team reported that many of the bones exhibited the traits identified by Turner as demonstrating cannibalism. But the Cowboy Wash team also found something else.

Photo by Paul Ermigiotti, courtesy Crow Canyon Archaeological Center

They uncovered a piece of coprolite human excrement deposited flagrantly within a hearth. Richard Marlar, at the time an associate professor of pathology at the University of Colorado Health Sciences Center, analyzed the coprolite and found evidence of human myoglobin, a muscle protein. Myoglobin isn't found in the intestinal tract unless it gets there in a meal.

"We were able to demonstrate, though his work, that this coprolite deposited in this hearth, by someone clearly making a statement, contained human muscle tissue," says Patricia Lambert, a bioarchaeologist who was part of the Cowboy Wash team. "It was kind of a complete package of evidence."

While the evidence seemed incontrovertible, there were still questions. What had led to these extreme activities? Was it the work of psychopathic death cults, as suggested by Turner and others? And furthermore, every example of possible cannibalism uncovered thus far had occurred before the 13th century; what, if anything, could these macabre incidents have had to do with the Mesa Verde region's mysterious depopulation a century later?

At Castle Rock, Kuckelman found many of the answers. In a 2002 American Antiquity article, she reported that Crow Canyon crews at that dig had discovered bones scarred with cutting and anvil marks, marrow-rich bones splintered, skulls that had been heated and cracked open, and bone fragments with polishing at their tips. While there was no telltale coprolite, she argued that there were enough similarities to link these remains to Turner's findings and those at Cowboy Wash essentially connecting Castle Rock with direct evidence of

cannibalism.

Then Kuckelman went a step further. Not only had she found evidence of cannibalism, she placed it in a very specific context: an archaeologically and perhaps historically verifiable battle that occurred at the end of the ancestral Pueblo people's occupation of the Mesa Verde region.

Between droughts and famine, warfare and cannibalism, it was starting to look like the 13th-century Mesa Verde region wasn't such a peaceful place after all.

Mistakes of the past

Kuckelman looks uncomfortable sitting in her office at Crow Canyon headquarters, surrounded by bookshelves crowded with such volumes as Digging in the Southwest and Gray's Anatomy, desktops cluttered with papers, Post-it notes and dirty trowels. In the mid-afternoon sunlight, her feet swing her swivel chair back and forth, her hands fly through the air. It's as if each part of her wants to be back in the wide-open spaces of Goodman Point Pueblo, the endlessly fascinating jigsaw puzzle through which she meandered all morning. But finally, after hours of technical explanations and precisely worded disclaimers, her mind is relaxing. Her words flow faster and more easily as she puts her carefully constructed story together.

Meticulously, she lays out her evolving explanation, a picture she's been building for decades, borne from her years exploring places like Castle Rock and Goodman Point Pueblo, honed by the opinions of scholars, excavators and Native Americans. It's a heady, complicated story, one far too complex and subtle not to mention unpleasant to fit easily into prepackaged Mesa Verde birthday brochures or centennial speeches. But it's a story that's slowly seeping out to the public, as Crow Canyon, through its community education programs, convinces more and more people to sift through the dirt alongside Kuckelman and other major players in the field.

Kuckelman's explanation begins in the mid-13th century, a time when the Mesa Verde region was in the midst of population growth like never before. People were flowing into the area, lulled by good weather and fertile fields.

But then the weather turned bad, and the region balkanized. Villages turned jealous eyes on their neighbors' resources, protectively hoarding their own crops and streams. Communities became fortresses built into cliff walls at Mesa Verde and around outcroppings at Castle Rock.

By the time the rains stopped, in 1276, as fields withered and children starved, people had turned to violence, raiding the stores of other villages, defending their remaining food with their lives. To frighten their enemies, they may have resorted to desperate, macabre measures, like cannibalism. Fertile lands still existed nearby, lands for which families might have pulled up stakes and relocated in lean times past. But now the area was too crowded, communities were too sedentary and interconnected, there was too much danger in the land. As the century drew to a close, the only choice was to begin leaving the region, traveling as families, as villages, to the south.

Those who were left behind found themselves in a world too fragmented, too depopulated, to continue the old ways, the religious rituals and kinship systems; they'd reached a point of no return. Soon they were gone, too, moving across the desert, looking for someplace they could start fresh.

It's just a story, Kuckelman knows, and a partial one at that. And even if it's completely accurate and she's worked long enough in the region to know that no theory can be taken as fact many questions about the final migration remain.

Working to answer these questions is critical, and not just as a capstone to a 100-year-birthday celebration. Consider: Population levels spiraling out of control. Water battles fracturing communities. Devastating climate changes, and societies too entrenched in their ways to do anything about it. Wars spurred by dwindling natural resources. People willing to resort to horrific violence over a piece of land. Towns, cities, societies laid to waste by natural disasters.

This is the picture Kuckelman paints of the past, but it's also a picture she sees in the newspapers every day. Seven hundred years from now, will archaeologists ponder the ruins of 21st-century cities, wondering what happened to their occupants?

Kuckelman crosses her arms and looks out the window of her office, gazing across the parched, inhospitable landscape. She chooses her words carefully.

"You cannot retreat to an ivory tower. What professional archaeologists need to keep in mind is that we are not just doing this for ourselves," she says. "I think it is particularly timely to be learning about a pretty large group of people, a society, that clearly made some pretty pivotal decisions about living in a particular landscape.

"We need to learn about how resources can be mismanaged and really cause devastating problems down the road. If you don't manage resources carefully and thoughtfully, you may be sowing the seeds of your own downfall."

A version of this story originally appeared in Westword, Denver's alternative newsweekly.